This invention relates to an accumulator usable in an automotive transmission to absorb hydraulic forces associated with clutch engagement actions.
An existing automatic overdrive transmission used in Ford automobiles has a liquid accumulator that comprises a stepped cavity machined into the transmission housing; a stepped piston is slidably disposed within the cavity for linear movement in response to liquid forces introduced into opposite ends of the cavity.
A variable throttle pressure is applied to the large bore end of the cavity and also to a liquid clutch. As the pressurized liquid pressures the clutch plates toward the clutch-engaged drive mode it also exerts a liquid force on the piston in the accumulator; the piston is moved to enlarge the liquid volume in the large bore end of the cavity, thereby absorbing some of the shock forces that would otherwise produce a jerky (non-smooth) vehicle speed change.
In the existing transmission the transmission housing is formed of aluminum, whereas the stepped piston is formed of a thermoplastic material. The piston has elastomeric O-ring seals slidably engaged with the aluminum cavity surfaces.
Some leakage problems have been encountered with the existing arrangement. The aluminum housing thermally expands at a higher rate than the plastic piston; as the temperature rises within the accumulator cavity the clearances between the piston side surfaces and cavity side surface increase, thereby tending to permit leakage of the pressurized liquids across the O-ring seals.
The O-ring seals compensate to some extent for the increased clearances between the piston and cavity side surfaces. However, in some cases the O-ring seals have not been fully effective. Also, the high temperatures over time tend to degrade the elastomeric seals, i.e., by destroying the plasticizers. The O-ring seals also tend to become pinched in the piston-cavity crack interfaces and to be abraded by contact with the relatively hard aluminum surfaces. Service life of the O-ring seals is generally much shorter than the life of the transmission in which they are installed.